The current standard, UL-voltage-rated cable is 600 volts. A voltage rating of 7,500 volts is not uncommon for industrial, utility or outside applications. However, when the insulation of the cable must have sufficient flame retardancy, or when it contains fillers and/or stabilizers for high-temperature operation (.gtoreq.150.degree. C.), high voltage capability is difficult to achieve. In addition, the wall thickness of present high-voltage motor lead wire (UL-style 3499) is 0.156 inches.
The present invention can provide a voltage rating that is greater than 1,000 volts, with the wire having a small wall thickness, one of approximately between 0.045 to 0.090 inches. Such results are achieved by introducing a semiconductive layer between the insulation and the wire core conductor. This cable article is most useful for electric motors, but this invention can also be applied to coils, transformers, generators and power supplies.
The semiconductive layer of the inventive cable has a nominal resistance of approximately 5,000 ohms. Applicant discovered that semiconductive materials with resistances of 1,000-5,000 ohms per square worked very well, but there were also successful test results with semiconductive layers over a wide range of resistances. The semiconductive layer can be extruded or tape-wrapped about the conductive core. For electric motors, the cable size is in an approximate range of between 8 AWG and 4/0 AWG. The semiconductive layer can comprise a carbon-impregnated polymer such as a carbon-impregnated polyethylene; a carbon-impregnated woven (or non-woven) fibrous-tape, or an ethylene-propylene-diene terpolymer (EPDM).